1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for a broadcasting television camera, a movie camera, a video camera, a digital still camera, a monitoring camera, and a silver-halide film camera, for example.
2. Description of the Related Art
In recent years, a zoom lens having a wide angle of field, a high zoom ratio, and high optical performance is required for an image pickup apparatus such as a television camera, a movie camera, a silver-halide film camera, a digital camera, or a video camera. There has been known, as a zoom lens having a wide angle of field and a high zoom ratio, a four-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power that is used for zooming, a third lens unit having a negative refractive power that is used to correct image plane variation accompanying zooming, and a fourth lens unit having a positive refractive power that is used for image formation (Japanese Patent Application Laid-Open Nos. H09-015501 and 2004-341237).
In Japanese Patent Application Laid-Open No. H09-015501, there is disclosed a zoom lens having a zoom ratio of approximately 8 and a field angle of photography at a wide angle end of approximately 87°. In Japanese Patent Application Laid-Open No. 2004-341237, there is disclosed a zoom lens having a zoom ratio of approximately 2.9 and a field angle of photography (an angular field of view) at a wide angle end of approximately 94°.
In the four-unit zoom lens having the structure described above, it is relatively easy to achieve the increased wide angle of field. However, in order to achieve both the high optical performance and the reduction in size of the four-unit zoom lens, it is important to appropriately set the arrangement of the refractive powers of the lenses. In particular, in the lens unit closest to an object side, an off-axis ray passes through a position farthest from the optical axis. Therefore, in order to achieve both the high optical performance and the reduction in size, it is important to appropriately set the refractive power and the structure of the lens unit closest to the object side. In addition, the lens unit closest to the image side plays a role of determining sizes of all lenses closer to the object side than the lens unit. Therefore, in order to achieve both the high optical performance and the reduction in size, it is important to appropriately set the lateral magnification of the lens unit closest to the image side. Unless those conditions are approximately set, it becomes difficult to obtain the zoom lens having the wide angle of field, the high zoom ratio, and the high optical performance over the entire zoom range.
The zoom lens disclosed in each of Japanese Patent Application Laid-Open Nos. H09-015501 and 2004-341237 is assumed to be applied to a relatively small image pickup element for a television camera. When the zoom lens is applied to an image pickup element that has been increased in size in recent years, the lens is also increased in size in proportion to the enlargement of the image pickup element.